1. Field of the Invention
The present invention relates to a thermistor changing a resistance value depending on temperature, and more particularly to a surface-mounted thermistor being mounted on a PCB (printed circuit board) and having a function of protecting other circuit elements.
2. Description of the Related Art
Among conductive materials, some materials change their specific resistances depending on temperature. A resistance element made using such materials is generally called ‘a thermistor’, which are representatively classified into an NTC (negative temperature coefficient) resistance element whose resistance decreases according to the rise of temperature and a PTC (positive temperature coefficient) resistance element whose resistance increases according to the rise of temperature.
In particular, the PTC resistance element passes a current at a low temperature such as a normal temperature since its resistance is low, while it interrupts a current if a temperature of the material is increased due to an excess current or a surrounding temperature is increased because its resistance increases as much as 1,000 to 10,000 times of its origin state. Thus, the PTC resistance element is commonly used as an element being mounted on a PCB for protecting other circuit elements by restraining an excess current.
Meanwhile, since various circuit elements are mounted on the PCB, the thermistor suffers from many restrictions in its mounting location and structure, particularly in the trend of these days, as described below in brief.
The thermistor is generally configured so that electrodes are respectively laminated on upper and lower surfaces of a layer made of PTC material. The thermistor configured as above is mounted by soldering an electrode, formed on the lower surface of the thermistor, to an electrode pad previously formed on a PCB surface. At this time, a separate wire is required for connecting the electrode formed on the upper surface of the thermistor to the electrode pad on the PCB, which also needs a relevant procedure and a space on the PCB. Thus, in order to avoid such disadvantages, the electrode on the lower surface of the thermistor is formed not on the entire area of the lower surface but on a region except a partial portion, and a metal pattern separated from the electrode on the lower surface is formed in the partial portion. The metal pattern and the upper electrode are electrically connected through a side of the PTC material layer. Then, when the thermistor is mounted on the PCB, the electrode and the metal pattern on the lower surface of the thermistor may be soldered to respective electrode pads, thereby not requiring a separate wire or space.
However, such a thermistor has some problems as follows.
First, a so-called Tombstone or Manhattan phenomenon occurs. When a thermistor is mounted on the PCB, the thermistor in which a solder is coated on the metal pattern and the electrode on the lower surface thereof is arranged on the electrode pad, and then the solder is reflowed by heating. Due to the applied heat, the PTC material and the electrode material of the thermistor are expanded. However, they have different thermal expansion coefficients, and in particular the thermistor having a configuration that the metal pattern and the electrode on the upper surface are connected through a side of the PTC material layer is structurally asymmetric, so right and left stresses are not uniform, thereby making the thermistor become inclined on the plane of PCB. As a result, physical and electrical reliability of the solder is significantly deteriorated. In order to lessen these problems, U.S. Pat. No. 6,380,839 suggested a thermistor structure in which thermal stress relief areas are formed in the electrodes on the upper and lower surfaces, but it does not give a fundamental solution.
In addition, the connection between the electrode on the upper surface and the metal pattern on the lower surface through a side of the PTC material layer drops physical and electric reliability. That is to say, the side of the PTC material layer is seriously influenced by an expansion pressure of PTC materials due to the heat applied during the solder reflow and due to the temperature increased while the thermistor is used, and if such a stress makes a crack in the connection portion formed on the side of the PTC material layer, this crack is propagated along the side of the PTC material layer, thereby probably cutting the electric connection.
In the prior art including the above U.S. patent, the thermistor is manufactured as follows. First, a plurality of long slits are formed in parallel on a sheet configured that metal films such as aluminum foils are coated on both sides of a PTC material layer, and then the electrodes or the metal patterns on the upper and lower surfaces are electrically connected through the slits. This slit becomes the aforementioned side of the PTC material layer later. A desired electrode pattern is formed between the slits, and then the processes such as solder resist coating and solder coating are conducted to successively make a plurality of thermistors between the slits. Finally, the sheet is cut in a direction perpendicular to the slits, thereby mass-producing thermistors of the same configuration.
However, if such long slits are formed in the sheet, a portion between the slits may droops down due to the gravity during the manufacturing process or be twisted due to the heat generated in the process. This results in that the pattern is inexactly formed in the electrode patterning process or the solder resist coating process, thereby increasing a failure rate.